1. Field of the Invention
The present invention relates to a pressure-sensitive adhesive composition required to have durability and light leakage preventive ability for an optical film and to a pressure-sensitive adhesive optical film including an optical film and a pressure-sensitive adhesive layer that is made from the pressure-sensitive adhesive composition on at least one side of the optical film. The invention also relates to an image display such as a liquid crystal display and an organic electroluminescence (EL) display, using the pressure-sensitive adhesive optical film. The optical film may be a polarizing plate, a retardation plate, an optical compensation film, a brightness enhancement film, a laminate thereof, or the like.
2. Description of the Related Art
The image-forming mode of liquid crystal displays or the like essentially requires polarizing elements to be placed on both sides of a liquid crystal cell, and generally polarizing plates are attached thereto. Besides polarizing plates, a variety of optical elements have been used for liquid crystal panels to improve display quality. For example, there are used retardation plates for prevention of discoloration, viewing angle expansion films for improvement of the viewing angle of liquid crystal displays, and brightness enhancement films for enhancement of the contrast of displays. These films are generically called optical films.
When optical members such as the optical films are attached to a liquid crystal cell, pressure-sensitive adhesives are generally used. Bonding between an optical film and a liquid crystal cell or between optical films is generally performed with a pressure-sensitive adhesive in order to reduce optical loss. In such a case, a pressure-sensitive adhesive optical film including an optical film and a pressure-sensitive adhesive layer previously formed on one side of the optical film is generally used, because it has some advantages such as no need for a drying process to fix the optical film.
The pressure-sensitive adhesive is required to have some characteristics. In some cases, for example, if in the process of bonding an optical film to a liquid crystal cell, they are misaligned or foreign matter is caught on the bonding surface, the optical film should be separated from the liquid crystal panel so that the liquid crystal cell can be recycled. For this separation process, the pressure-sensitive adhesive should have re-peelability (reworkability) such that the optical film can be easily peeled from the liquid crystal panel with no adhesive residue. Particularly in recent years, thin liquid crystal panels using chemically-etched glass plates are frequently used together with conventional panel manufacturing processes, and it has become difficult to subject optical films from the thin liquid crystal panels to reworking or processing. The pressure-sensitive adhesive is also required to have processability such that it can be processed without causing adhesive stain or dropout after it is formed. In addition, the pressure-sensitive adhesive is required not to cause any defect in durability tests by heating, moistening and so on, which are generally performed as accelerated environmental tests. Also, the pressure-sensitive adhesive is required to prevent light leakage caused by dimensional change generated in the pressure sensitive adhesive layer in the film.
A conventionally proposed method to solve the problems with the reworkability of liquid crystal panels includes adding a plasticizer or an oligomer component to an acrylic polymer used as a base polymer for an acrylic pressure-sensitive adhesive (see Japanese Patent Application Laid-Open (JP-A) No. 2003-329837). However, such an acrylic pressure-sensitive adhesive cannot provide satisfactory reworkability or processability for the thin liquid crystal panels.
Other than the above, proposed acrylic pressure-sensitive adhesives for optical films include an acrylic pressure-sensitive adhesive including an acrylic polymer produced using alkyl (meth)acrylate, a monomer having a hydroxyl group in the molecule and a monomer having a functional group such as a carboxyl, amide or amino group in the molecule as monomer components (see JP-A No. 2004-091499 and JP-A No. 2004-091500); an acrylic pressure-sensitive adhesive including an acrylic polymer produced using alkyl (meth)acrylate, a nitrogen containing monomer such as a imide-group containing monomer and amide-group containing monomer and further containing peroxide and isocianate compound (see JP-A No. 2007-138147); and a pressure-sensitive adhesive including an acrylic polymer produced using aromatic ring-containing monomer (see Japanese Patent Publication of Examined Application No. 62-23287). However, none of these acrylic pressure-sensitive adhesives disclosed in the patent literatures can provide satisfactory reworkability or processability, although they can provide improved durability and adhesiveness.
On the other hand, methods of adding a plasticizer or an oligomer to a pressure-sensitive adhesive is also proposed to reduce light leakage from an image display produced with optical films (see Patent Documents 6: Japanese Patent No. 3594206 and 7: Japanese Patent No. 3533589). However, these methods have a problem with workability or durability.
A pressure-sensitive adhesive composition for improvements in durability and reworkability and remediation of light leakage is proposed, which includes a copolymer of an acrylate eater, an aromatic ring-containing monomer, and a hydroxyl group-containing monomer as a base (see Patent Documents 8: JP-A No. 2007-138057 and 9: JP-A No. 2007-138056). However, such a copolymer does not have satisfactory durability, because of its low molecular weight and high degree of dispersion. Such a pressure-sensitive adhesive composition is intended to reduce staining or an adhesive residue during re-peeling, but it is not easily peelable and therefore not practically satisfactory.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2003-329837    Patent Document 2: JP-A No. 2004-091499    Patent Document 3: JP-A No. 2004-091500    Patent Document 4: JP-A 2007-138147    Patent Document 5: Japanese Patent Publication of Examined Application No. 62-23287    Patent Document 6: Japanese Patent No. 3594206    Patent Document 7: Japanese Patent No. 3533589    Patent Document 8: JP-A No. 2007-138057    Patent Document 9: JP-A No. 2007-138056